class DataService:
    def __init__(self):
        self.data = {}

    def add_data(self, key, value):
        """标准化数据接入方法"""
        self.data[key] = value

    def get_data(self, key):
        """获取数据"""
        return self.data.get(key)

    def list_data(self):
        """列出所有数据"""
        return self.data